Electronic heat control for dryer



1968 A. NIEWYK ET AL 3,419,708

ELECTRONIC HEAT CONTROL FOR DRYER Filed April 6. 1966 INVENTORS in //70/? Mewy c/ames Z. "C0/7/2e// 6290/ e M ens M Y ATTORX YS United States Patent ELECTRONIC HEAT CONTROL FOR DRYER Anthony Niewyk, St. Joseph, James L. McConnell,

Stevensville, and George H. Myers, St. Joseph,

Mich., assignors to Whirlpool Corporation, Benton Harbor, Mich., a corporation of Delaware Filed Apr. 6, 1966, Ser. No. 540,554 6 Claims. (Cl. 219501) This invention relates generally to solid state electronic temperature control devices, and more specifically to temperature control devices for use in conjunction with control circuitry heretofore regulated by thermostats and wherein the use of an accurate electronic temperature control affords greater flexibility in the selection of heaters and improved regulation of the cyclic control thereof.

An illustrative embodiment of the present invention is shown in combination with a home appliance such as a clothes dryer; however, the control devices of the present invention find application in various types of apparatus where accurate close temperature control is required. For example, the temperature control of the present invention could be used in conjunction with cooking ranges and ovens to accurately control the temperature thereof. Furthermore, the temperature control device of the present invention could be used in conjunction with die casting and plastic molding apparatus to control mold temperature.

Generally, domestic clothes dryers have a treatment zone or chamber wherein wet clothes which are to be dried are placed. A supply of heated air is delivered to the treatment chamber, whereupon the heated air becomes laden with moisture and is exhausted from the treatment chamber. The temperature of the air which is exhausted from the treatment chamber is directly related to the relative dryness of the clothes being dried and the program or cycle of drying. Thus, when heat is added to the air stream initially, the temperature rises rather quickly and then levels ofl? as the added thermal energy is utilized as heat of evaporization. When the contents of the treatment chamber are dry the temperature rises sharply.

Should thermal energy continue to be added to the treatment chamber after the clothes are substantially dry, the clothes may be scorched or otherwise damaged.

By utilizing accurate fast acting temperature responsive control means, large capacity heaters may be utilized thereby cutting down drying time without increasing the risk of scorching.

It is an object of the present invention to provide a temperature control device which will deliver 90% or more of the available heat energy to a treatment chamber of a clothes dryer during certain periods of the drying cycle and which will deliver approximately 50% of the available heat energy to the treatment chamber during other periods of the drying cycle.

Still another object of the present invention, particularly from a commercial standpoint, is to provide improved temperature control devices for use in conjunction with domestic clothes dryers which are relatively inexpensive to manufacture and which provide a high degree of reliability and accuracy.

A feature of the present invention is the use of a temperature dependent variable resistor which is positioned so as to be in contact with the air which is exhausted from the treatment chamber of the clothes dryer to sense the temperature thereof.

Still another feature of the present invention is the use of a single gating circuit to control a pair of silicon controlled rectifiers which, in turn, control the electric current flow delivered to a heating element.

These and other objects and novel features will be realized and more fully understood from the following Patented Dec. 31, 1968 detailed description when taken in conjunction with the accompanying drawings in which like reference numerals throughout the various views of the'drawings are intended to designate similar elements or components.

On the drawings:

FIGURE 1 is a somewhat diagrammatic illustration of a domestic clothes dryer showing particularly the flow path of heated air therethrough; and

FIGURE 2 is a schematic wiring diagram showing the component parts of a temperature control device which is constructed in accordance with the principles of this lnvention.

As shown on the drawings:

As seen in FIGURE 1, a clothes dryer 10 has an inlet 11 which is connectable to a blower for forcing air through an air passage 12. One or more electric heating elements 13 are positioned within the passage 12 to heat the air passing therethrough. The passage 12 has an opening 14 which is in communication with the treatment zone of the dryer 10.

Also in fiuid communication with the treatment zone is an outlet 16 whereby the heated air delivered to the treatment zone may be exhausted to atmosphere. The treatment zone of a clothes dryer may take many forms, including tumbling drums or receptacles, or other material agitating means. For purposes of this disclosure the treatment zone of the dryer 10 may be considered the total volume of space in the air circuit disposed between the inlet 14 and outlet 16.

To control the amount of current delivered to the heating element 13, a temperature control circuit 17 is housed within the dryer 10 and is electrically connected to the heating element 13. Furthermore, a temperature dependent variable resistor 18 is positioned so as to be in contact with the exhausted air from the treatment zone of the dryer before the heated air is exhausted to atmosphere through the outlet 16.

For a better understanding of the temperature control device of the present invention reference is now made to FIGURE 2. The heating element 13 is divided into two sections 13 and 13'. The heating element 13 is connected to a source of alternating current voltage through terminals 2t) and 21, while the heating element 13 is connected to the source of alternating current voltage through the terminal 20 and through an electronic switching circuit 22.

The electronic switching circuit 22 includes a pair of silicon controlled rectifiers 23 and 24 which are connected in parallel to one another and in series with the heating element 13. When the silicon controlled rectifier 23 is rendered conductive, current will flow from the terminal 21 through the rectifier 23, through a resistor 26 and through the heating element 13 of the terminal 20.

A capacitor 27 has one end thereof connected to a circuit point 28 between the resistor 26 and rectifier 23 and the other end thereof is connected to the gate electrode of the silicon controlled rectifier 24. Also connected to the gate electrode of the silicon controlled rectifier 24 is one lead of a resistor 29, While the other lead of the resistor 29 is connected to the cathode electrode of the silicon controlled rectifier 24 and to the heating element 13.

A triggering circuit 30 is provided for generating triggering pulse signals from a direct current voltage delivered thereto by a converter circuit 31. The AC to DC converter circuit 31 has a resistor 32, a diode 33 and a capacitor 34 connected in series between the terminals 20 and 21 to develop a direct current voltage across the capacitor 34. A current limiting resistor 36 has one end thereof connected to a circuit point 37 and the other end thereof is connected to a regulator tube 38. The regulator tube 38 is connected in parallel with the capacitor 34 and serves to maintain the direct current voltage output of the converter 31 at a substantially constant predetermined value.

The direct curent voltage from the converter 31 is de livered to the triggering circuit 30 whereupon the direct current voltage is converted into a series of pulse signals which are applied to the gate electrode of the silicon controlled rectifier 23.

A manually variable resistor 40 has one end thereof connected to the output of the converter 31 and the other end thereof is connecetd to one end of the temperature responsive variable resistor 18 which, in turn, is connected to the other output terminal of the converter 31. The variable resistors 18 and 40 form a voltage divider network for the direct current voltage received from the converter 31. The resistor 40 may be used to select the minimum drying temperature delivered to the treatment chamber.

A capacitor 41 is connected in parallel with the temperature responsive resistor 18 and will tend to become charged to a voltage substantially equal to the voltage across the resistor 18. A switching diode 42 has one end thereof connected to a circuit point 43 and the other end thereof connected to the gate electrode of the silicon controlled rectifier 23 through a resistor 44. Preferably, the switching diode 42 is a PNPN triggering diode which has a breakover voltage of 22 volts. When the capacitor 41 charges to a value of 22 volts, the switching diode 42 becomes conductive thereby discharging the capacitor 41 through the resistor 44 and the cathode gate circuit of the silicon controlled rectifier 23.

In order to assure conduction of the silicon controlled rectifier 23 early in the half cycle for which it conducts, it is contemplated by this invention that the RC time constant of the resistors 18 and 40 and the capacitor 41 be approximately /5 of the time interval of one-half cycle of the alternating current voltage applied between terminals 20 and 21. Therefore, during the time interval when terminal 20 is positive with respect to terminal 21, the silicon controlled rectifier 23 will be rendered conductive early in the cycle and no later than /5 of the time of one-half cycle.

In operation, the source of alternating current voltage is applied to terminals 20 and 21 through a timing switch or the like which controls the duration of the timing cycle of the dryer 10. During this time, the heating element 13 is continuously energized by the alternating current. However, the heating element is selectively energized depending upon the temperature within the treatment chamber of the dryer at particular times during the drying cycle.

A portion of the alternating current voltage between terminals and 21 is rectified and held substantially constant by the regulator tube 38 at the output of the converter circuit 31. The regulated output voltage of the converter circuit 31 is then delivered to the voltage divider network consisting of the manually variable resistor 40 and the temperature responsive variable resistor 18.

The resistor 18 is positioned within the dryer 10 so as to be in contact with the exhausted air from the treatment chamber. Therefore, at the beginning of the drying cycle the exhausted air from the treatment chamber is relatively cool and the resistance value of the resistor 18 is high. This high value of resistance of the resistor 18 will cause a substantial portion of the output voltage from the regulator circuit 31 to be developed across the resistor 18. This voltage is preferably in excess of 22 volts and will charge the capacitor 41 until such time that the charge across the capacitor reaches a value of 22 volts.

At that instant, the breakover voltage of the switching diode 42 is reached and the switching diode 42 will be rendered conductive and discharge the capacitor 41 through the resistor 44 and the cathode gate circuit of the silicon controlled rectifier 23.

When the capacitor 41 has discharged to a voltage which is less than the threshold voltage which is required to maintain the switching diode 42 conductive, the switching diode 42 will again revert to its non-conductive state, and the capacitor 41 will again become charged by the voltage developed across the resistor 18.

When terminal 20 is positive with respect to terminal 21, and with trigger pulse signals generated by the triggering circuit 30, the silicon controlled rectifier 23 is rendered conductive. The current through the silicon controlled rectifier 23 passes through the resistor 26 and through the heating element 13 for generating heat therefrom. The heat energy from the heating element 13 is added to the heat energy from the heating element 13' and delivered to the treatment zone of the dryer 10. It will be understood that the resistor 26 has a relatively low resistance value as compared to the resistance value of the heating element 13.

As the silicon controlled rectifier 23 conducts, a portion of the current passes through the resistor 29 to thereby place a positive charge on the capacitor 27 at the terminal connected to the gate electrode of the silicon controlled rectifier 24. At the end of the positive half cycle, terminal 20 becomes negative with respect to terminal 21 thereby reverse-biasing the silicon controlled rectifier 23 and rendering it nonconductive. On the other hand, silicon controlled rectifier 24 is forward-biased during that time and the positive charge on the capacitor 27 is discharged through the resistor 29 and the cathode gate circuit of the silicon controlled rectifier 24 to render the silicon controlled rectifier 24 conductive. The resistance value of resistor 29 is selected so as to shunt a portion of the charged current from the capacitor 27 and limit the amount of current which passes through the cathode gate circuit of the silicon controlled rectifier 24. Therefore, during the negative half cycle of the applied alternating current voltage, the silicon controlled rectifier 24 is rendered conductive to allow curent flow through the heating element 13.

When the temperature within the treatment zone has reached a predetermined value, the temperature dependent resistor 18 decreases its resistance value which will cause less than 22 volts to be developed thereacross. The reduced voltage will cause the capacitor 41 to become charged at a correspondingly lesser voltage, and the switching diode 42 will remain in its nonconductive state. Therefore, when terminal 20 becomes positive with respect to terminal 21 the silicon controlled rectifier 23 will remain nonconductive since no triggering signals are applied to the gate electrode thereof. Also, since the silicon controlled rectifier 23 remains nonconductive, the capacitor 27 will not obtain a charge thereacross and the silicon controlled rectifier 24 will also remain nonconductive during the preceding negative half cycles of the applied alternating current.

It can be seen therefore that the silicon controlled rectifier 24 is slaved to the silicon controlled rectifier 23. That is, the silicon controlled rectifier 24 will remain nonconcluctive during the negative half cycles immediately preceding the positive half cycles for which the silicon controlled rectifier 23 was nonconductive, and the silicon controlled rectifier 24 will be rendered conductive during the negative half cycles preceding the positive half cycles for which the silicon controlled rectifier 23 was conductive.

Therefore, the present invention has provided a simple and efiicient electronic switching circuit for controlling the temperature of drying air delivered to the treatment chamber of a clothes dryer.

As an alternate embodiment, the circuit of FIGURE 2 may be modified so as to use a Zener diode in place of regulator tube 38, a unijunction transistor in place of switching diode 42, and a bidirectional switch, such as a triac, in place of silicon controlled rectifiers 23 and 24, the resistors 26 and 29 and the capacitor 27. Such a modified circuit would function no differently from that described above for the circuit of FIGURE 2.

It should be understood that we wish to embody within the scope of the patent warranted hereon all such modications as may be reasonably included within the scope of our contribution to the art.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A temperature control device comprising: a source of alternating current voltage having a pair of current delivery lines; an electrically energized heating element having one end thereof connected to one line of said source; first and second electronic switching means connected in parallel to one another and in series with said heating element for connecting said heating element to the other line of said source through said first switching means during the positive half cycles from said source and through said second switching means during the negative half cycles from said source; converter means connected to the lines of said power source for changing a portion of the alternating current voltage to a direct current voltage; temperature responsive triggering means connected between said converter means and said first switching means for generating triggering signals from said direct current voltage of said converter means when the temperature generated by said heating element is less than the desired temperature, thereby rendering said first switching means conductive during the positive half cycles from said source; and means connected between said first and second switching means for rendering said second switching means conductive during the negative half cycle from said source immediately after conduction of said first switching means during the previous positive half cycle of said source, and for maintaining said second switching means nonconductive during the negative half cycles when said first switching means is nonconductive during the preceding positive half cycle.

2. The temperature control device of claim 1 wherein said switching means includes; first and sec-0nd silicon control rectifiers each having anode, cathode, and gate electrodes, the cathode of said first silicon control rectifier being connected to the anode of said second silicon control rectifier and to said source, and the anode of said first silicon control rectifier being connected through a resistor to the cathode of said second silicon control rectifier and to one end of said heating element, and the gate of said first silicon control rectifier being connected to said triggering means for receiving triggering signals therefrom; and said last named means including a capaci tor connected between the anode of said first silicon control rectifier and the gate of said second silicon controlled rectifier, and a resistor connected between the gate of said second silicon control rectifier and the cathode of said second silicon control rectifier.

3. The temperature control device of claim, 1 wherein said temperature responsive triggering means includes; first and second variable resistors connected across the direct current voltage of said converter means to form a voltage divider network, said first resistor being manually variable and said second resistor being variable in response to changes in temperature from said heating element, said second resistor positioned in proximity to the heat energy generated by said heating element, a capacitor connected across said second resistor for receiving a voltage charge which is proportional to the voltage developed across said second resistor, and a switching diode having a predetermined breakover voltage, said switching diode connected between last said capacitor and said first switching means for rendering said first switching means conductive whenever the voltage charge across last said capacitor becomes greater than the predetermined breakover voltage of said switching diode.

4. A temperature control device for regulating the temperature within a treatment chamber of a clothes dryer, comprising: :an air inlet to the clothes dryer; a first air path connected between said inlet and the treatment chamber for delivering heated air to the treatment chamber; an air outlet from the clothes dryer; a second air passage connected between said air outlet and the treatment chamber for exhausting the heated air from the treatment chamber; a source of alternating current voltage including a pair of current delivery lines; an electrically energized heating element positioned within said first air passage for heating the air passing therethrough, said heating element having one end thereof connected to one line of said source; first and second electronic switching means connected in parallel to one another and in series with said heating element for connecting said heating element to the other line of said source to said first switching means during the positive half cycle from said source and through said second switching means during the negative half cycle from said source; converter means connected to the lines of said power source for changing a portion of the alternating current voltage to a direct current voltage; temperature responsive triggering means responsive to the temperature of heated air passing through said second air passage, said triggering means connected between said converter means and said first switching means for generating triggering signals from said direct current voltage of said converter means when the temperature sensed within said second air passage is less than a predetermined value thereby rendering said first switching means conductive during the positive half cycles from said source; and means connected between said first and said second switching means for rendering said second switching means conductive during the negative half cycle from said source immediately after conduction of said first switching means during the previous positive half cycle, and for maintaining said second switching means nonconductive during the negative half cycle when said first switching means is nonconductive during the preceding positive half cycle.

5. The temperature control system of claim 4 wherein said switching means includes: first and second silicon control rectifiers each having anode, cathode, and gate electrodes, the cathode of said first silicon control rectifier being connected to the anode of said second silicon control rectifier and to said source, the anode of said first silicon control rectifier being connected through a resistor to the cathode of said second silicon control rectifier and to one end of said heating elements, the gate of said first silicon control rectifier being connected to said triggering means for receiving trigger signals therefrom; and said last named means including, a capacitor connected between the anode of said first silicon control rectifier and the gate of said second control rectifier, and a resistor connected between the gate of said second silicon control rectifier and the cathode of said second silicon controlled rectifier.

6. The temperature control system of claim 4 wherein said temperature responsive triggering means includes: first and second variable resistors connected across the direct current voltage of said converter means to form a voltage divider network, said first resistor being manually variable and said second resistor being variable in response to changes in temperature through said second :air passage, a capacitor connected across said second resistor for receiving a voltage charge which is proportional to the voltage developed across said second resistor, and a switching diode having a predetermined breakover voltage, said switching diode being connected between said capacitor and said first switching means for rendering said switching means conductive whenever the voltage across said capacitor becomes greater than the predetermined breakover voltage of said switching diode.

No references cited.

BERNARD A. GILHEANY, Primary Examiner. FRED E. BELL, Assistant Examiner. 

1. A TEMPERATURE CONTROL DEVICE COMPRISING: A SOURCE OF ALTERNATING CURRENT VOLTAGE HAVING A PAIR OF CURRENT DELIVERY LINES; AN ELECTRICALLY ENERGIZED HEATING ELEMENT HAVING ONE END THEREOF CONNECTED TO ONE LINE OF SAID SOURCE; FIRST AND SECOND ELECTRONIC SWITCHING MEANS CONNECTED IN PARALLEL TO ONE ANOTHER AND IN SERIES WITH SAID HEATING ELEMENT FOR CONNECTING SAID HEATING ELEMENT TO THE OTHER LINE OF SAID SOURCE THROUGH SAID FIRST SWITCHING MEANS DURING THE POSITIVE HALF CYCLES FROM SAID SOURCE AND THROUGH SAID SECOND SWITCHING MEANS DURING THE NEGATIVE HALF CYCLES FROM SAID SOURCE; CONVERTER MEANS CONNECTED TO THE LINES OF SAID POWER SOURCE FOR CHANGING A PORTION OF THE ALTERNATING CURRENT VOLTAGE TO A DIRECT CURRENT VOLTAGE; TEMPERATURE RESPONSIVE TRIGGERING MEANS CONNECTED BETWEEN SAID CONVERTER MEANS AND SAID FIRST SWITCHING MEANS FOR GENERATING TRIGGERING SIGNALS FROM SAID DIRECT CURRENT VOLTAGE OF SAID CONVERTER MEANS WHEN THE TEMPERATURE GENERATED BY SAID HEATING ELEMENT IS LESS THAN THE DESIRED TEMPERATURE, THEREBY RENDERING SAID FIRST SWITCHING MEANS CONDUCTIVE DURING THE POSITIVE HALF CYCLES FROM SAID SOURCE; AND MEANS CONNECTED BETWEEN SAID FIRST AND SECOND SWITCHING MEANS FOR RENDERING SAID SECOND SWITCHING MEANS CONDUCTIVE DURING THE NEGATIVE HALF CYCLE FROM SAID SOURCE IMMEDIATELY AFTER CONDUCTION OF SAID FIRST SWITCHING MEANS DURING THE PREVIOUS POSITIVE HALF CYCLE OF SAID SOURCE, AND FOR MAINTAINING SAID SECOND SWITCHING MEANS NONCONDUCTIVE DURING THE NEGATIVE HALF CYCLES WHEN SAID FIRST SWITCHING MEANS IS NONCONDUCTIVE DURING THE PRECEDING POSITIVE HALF CYCLE. 